The objective of this study was to examine the techniques of selecting logging-while-drilling (LWD) tools for geosteering in unconventional reservoirs by examining the workflows and choices from case studies. When planning a horizontal well one of the most important decisions is choosing the measurement that will be used to steer. Which tool to select depends on the measurement contrast between the target formation and the surrounding formations, target thickness and most importantly what are the project objectives.
Judiciously choosing the correct measurement can help maximizing exposure within the target window and reduce trouble time and sidetracks. Steering within unconventional reservoirs is generally done using the simplest measurements possible, the measurements-while-drilling gamma ray (MWD GR). This is due to cost or lack of perceived need for additional measurements, or because GR gives enough information with the large amount of offset data that exists. We looked at several case studies where tools were selected by analyzing the offset for measurement contrast and forward modeling the planned well trajectory across the zone and exiting the top and base of the target window.
One case was a series of wells in the Olmos sand found in the coastal area of south Texas. The target is a higher-porosity layer within the Olmos “C” sand, which is approximately 10 to 12 ft thick with surrounding rock is that is tighter but will produce if fractured. The project objective was to drill wells to maximize exposure in the high-porosity layer, then hydraulically fracture the reservoir. The offset log data was forward modeled, then the best measurement that would achieve project objectives was chosen and the wells drilled.
The offset data was forward modeled and it showed that because of the symmetrical nature of the target window, an azimuthal measurement was needed. Both azimuthal GR or resistivity would work in this environment, but to distinguish between the tighter formation above and the target with a similar resistivity value a different measurement would be needed to have a unique measurement that could distinguish between the higher density rock above the target and lower density rock below the target, and an imaging density tool was selected to steer the well.
The wells were landed in the target zone using a conventional gamma ray (GR), and then geosteered during the lateral using real-time density images from an LWD tool. The images were used to measure formation dips, both within the target and after the trajectory was forced out of the zone by a subseismic fault. The formation dip was determined to plan the sail angle to allow for getting back in zone most efficiently.
Selecting the proper measurement by careful analysis beforehand allowed the wells to be steered successfully, which led to increased production compared to offset horizontals steered without an azimuthal measurement.